1. Field of the Invention
The present invention relates to a twin-clutch type transmission unit for an engine, and to a motorcycle including the same. More particularly, the present invention relates to a twin-clutch type transmission unit provided with two clutches, and to a motorcycle including the same.
2. Description of the Background Art
There is a known twin-clutch type transmission unit in which a first input shaft and a second input shaft disposed coaxially are provided respectively with a first clutch and a second clutch for transmission and interruption of input from an engine, a plurality of gear pairs for transmission are provided between the first and second input shafts and an output shaft, and the first clutch and the second clutch are alternately disengaged and engaged so as to perform a gear shift. An example of such twin-clutch type transmission unit is disclosed in the Japanese Patent Laid-open No. 2008-89064.
In general, the capacity of a clutch for gear shift is designed according to the capacity at the time of vehicle starting when the rotational acceleration is the greatest. However, a clutch capacity as high as that at the time of vehicle starting is not needed during normal vehicle running. In an existing above-mentioned twin-clutch type transmission unit, two sets of clutches for gear shift are used. Use of two sets of clutches disadvantageously results in increased clutch weight and enlarged clutch size. Therefore, reductions in the size and weight of the twin-clutch type transmission unit are being desired.
The present invention has been made to overcome the drawbacks of existing twin-clutch type transmission unit. Accordingly, it is one of the objects of the present invention to contrive reductions in the size and weight of a twin-clutch type transmission unit.